1. Field of the Invention
The present invention relates to an electrode assembly and a secondary battery having the same, and more particularly to an electrode assembly to which a ceramic layer is laminated and a secondary battery having the same.
2. Description of the Related Art
Generally, a secondary battery can be used repeatedly when it is charged, which is different characteristics from a primary battery. It serves as a main power of mobile apparatuses for communication, information processing, and audio/video. The main reason that the secondary battery recently comes into the spotlight and makes rapid growth is that it has a high energy density, a high output voltage, a low self-discharge ratio and a long life span, and has ultra light weight and is environmentally friendly.
Secondary batteries are classified into Ni-MH batteries, Li-ion batteries, and the like, based on electrode active materials. Particularly, the Li-ion batteries may be classified based on the type of electrolyte, for example, whether liquid electrolyte is used, or solid electrolyte or gel-type electrolyte is used. And, the secondary batteries are classified into various types, such as a can type, a pouch type, and the like, based on the shape of a can within which the electrode assembly is accommodated.
Since the Li-ion battery has an energy density per weight far superior to that of a primary battery, it is possible to embody an ultra light weight battery using the Li-ion battery. Also, an average voltage per cell of the Li-ion battery is 3.6V, which corresponds to three times of the average voltage of other secondary batteries, such as Ni—Cd battery or Ni-MH battery. Further, since the Li-ion battery has self discharge ratio of less than about 5% per month at 20° C. which is about ⅓ of the Ni—Cd battery or Ni-MH battery and it does not use heavy metal such as Cd or Hg, the battery is environmentally friendly, and can be charged and discharged over 1000 times in a normal condition. Accordingly, due to these advantages, the popularity of secondary battery rapidly grows, and recent progress has been made in the area of information technology.
In a conventional secondary battery, an electrode assembly, which includes a positive electrode plate, a negative electrode plate and a separator, is received in a can formed of aluminum or aluminum alloy. A cap assembly is installed at the top opening of the can, and the can is filled with an electrolyte and sealed, completing a bare cell. If the can is formed of aluminum or aluminum alloy as described above, the secondary battery has advantages that it can become light since aluminum is light in weight, and is prevented from being corroded even when used under high voltage conditions for a long time.
In order to complete a battery pack, the sealed bare cell is inserted in a hard pack, and is connected to a safety means such as positive temperature coefficient (PTC) device, thermal fuse and protective circuit module (PCM), and other battery accessories, or is sealed by using hot melt resin.
Meanwhile, a separator of the battery assembly being formed of an olefin group film prevents two electrodes from being short and the battery from being heated. However, when temperature of the battery suddenly increases due to an external heat transfer or the like, the temperature of the battery continues to increase for a predetermined time although minute through-holes of the separator are closed, so that the separator can be damaged.
Further, if the battery has a high capacity with a high density coating portion so as to have a high density of electrode plate, the electrolyte does not penetrate into the electrode plate, so that electrolyte injection speed of the battery becomes slower or the battery may not have enough electrolyte.
Further, if a large amount of current flows in the secondary battery in a short time period due to the high capacity of the battery, even though the through-holes of the separator are closed, the current cut-off does not decrease the temperature of the battery. Instead, the separator continues to melt by the heat already produced, so that the separator may be damaged, increasing the possibility of internal short circuit.
Accordingly, as it is required that the internal short circuit between electrodes is prevented even at a high temperature, the separator is formed of ceramic layers having porous membrane which are made of particles of the ceramic pillars coupled with thermo-stable binder.
That is, the ceramic layer is more efficient to prevent the internal short circuit, improving safety of the battery, as described above, and is free from being contracted or molten even at internal short-circuit since it is formed on the electrode plate as a coating. Further, the battery has satisfactory and high charging/discharging characteristics because of the use of ceramic powder having a high porosity, and increases an injection speed of the electrolyte since the ceramic powder absorbs the electrolyte rapidly.
As shown in FIG. 8, the conventional electrode assembly, to which the ceramic layer is applied, includes a positive electrode plate 12, a negative electrode plate 22, and a ceramic layer 30 which is formed on the positive electrode or negative electrode plate to prevent the positive electrode and negative electrode plates from being short-circuited and to permit only lithium ions to move therebetween. The positive electrode plate 12 includes a positive electrode collector 10 and a positive electrode coating portion 11 formed on a determined area of the positive electrode collector 10. The negative electrode plate 22 includes a negative electrode collector 20 and a negative electrode coating portion 21 formed on a determined area of the negative electrode collector 20. The positive electrode plate 12, negative electrode plate 22, and the ceramic layer 30 are laminated.
The ceramic layer 30 is uniformly formed on an electrode collector of the positive electrode plate 12 or the negative electrode plate 22 facing each other, and is formed on the electrode coating portion. Accordingly, while improving the stability of the battery by laminating the ceramic layer 30 on the electrode plate in the art, the ceramic layer 30 is laminated on the entire surface except non-coating portions of front and rear sides of the electrode plate where the electrode coating portion is not formed, resulting in decrease of production efficiency due to an increased material cost and the difficulty of quality control.